What is the use of a convex edge?

In the knife world quite a few claims are made regarding the advantages a convex edge may have over a straight edge. In this post I examine some of these claims.

A convex edge is thousands of years old

This claim is definitely true. Even in the bronze age people made blades with convex edges. Here is the (beautiful) empirical evidence.

In fact, a convex edge was the only type of edge people were able to make back then when sharpening metal. If you work free-hand, no matter how precise you try to be, your edge will always be somewhat convex. To create a fully straight edge you need a guided angle system like the Wicked Edge.

A convex edge is marketing 

This is true as well. We live in times where machines, sometimes in far-away countries, make products that don’t always deliver the quality they should. Old handcrafts are appreciated again. This, in combination with claims about the advantages of a convex edge, makes that they are (sometimes over)valued. And fine companies like Fallknives and Bark River surely don’t keep it secret that they put convex blades on their knives.

A convex edge is stronger

Stronger than a straight edge, that is. This claim is often followed by “because it has a better geometry” or “because it has more steel behind it”.

However, this claim is rather meaningless. Stronger than what? The claim says little,  because it omits the angles of the edges we are comparing.

Yes, a convex edge that varies between 17 degrees and 22 degrees (22 degrees at the edge of the edge) is stronger than a straight edge of 17 degrees. However, it is not stronger than a straight edge of 22 degrees.

See the picture below. It shows a convex edge (black line) that has a 22 degree angle at the top and 17 degrees at the bottom. This convex edge is certainly stronger than a 17 degree straight edge (blue lines): the convex edge has more steel behind it. However, the same convex edge convex is not stronger than a straight edge of 22 degrees (red line). In this case the straight edge has more steel behind it.

Drawing1

A convex edge of 17 – 22 degrees is stronger than a straight edge of 17 degrees, because it has more material behind it. However, this convex edge is not stronger than a straight edge of 22 degrees: the 22 degree straight edge has more material behind it.

Yet the idea that a convex edge is stronger is appealing. This is sometimes called the pencil point theory. We all know that by rounding the point of a pencil the probability that it breaks off is much reduced.

This is similar to what we do when we put a micro-bevel on straight primary bevel. We still benefit a lot from the steeper primary bevel, but we make it less fragile by putting on a blunter micro-bevel.

In these cases we don’t just make the edge stronger. We sacrifice a bit of sharpness in favour of strength. In other words: we try to optimize the balance sharpness-strength.

A convex edge dulls slower

Some people think a convex edge dulls slower than a straight edge. But again, this depends on the angles.

A convex edge dulls slower than a straight edge only if it was duller in the first place. For example, if one convexes a straight edge of 17 degrees so that it becomes 22 degrees at the edge of the edge, this edge will indeed wear slower. See the picture below.

A convex edge from 17-22 degrees will dull slower than a straight edge of 17 degrees.

A convex edge from 17-22 degrees will dull slower than a straight edge of 17 degrees.

However, a convex edge that is 22 degrees at the edge of the edge does not keep its sharpness longer than a straight edge of 22 degrees. This is because the straight edge has more material right behind the edge. See the picture below.

Drawing2

A convex edge of 17-22 degrees does not dull slower than a straight edge of 22 degrees. This straight edge has more material behind it.

A convex edge doesn’t wedge

Well, at least not as much as a straight edge, so the claim goes. One of the most annoying features of an axe is that it may wedge. This is the reason everybody prefers a convex edge on their axe.

Wedging is also a big annoyance for cooks when cutting harder materials like carrots or onions. High-end kitchen knife blades are thin behind the edge. Some cooks go as far as recommending thinning out a knife every time it is sharpened.

Here indeed a convex edge has an advantage over a straight edge. A convex edge has a smaller part of the edge in contact with the material being cut than a straight edge. So there is less friction. This is illustrated by the picture below (inspired by this).

Drawing4

A convex edge (left) and a straight edge cutting a hard material (blue). The red lines indicate the areas of contact between the edges and the subtrate. The convex edge has less area in contact with the material at any one time.

Conclusion

My conclusion is that some of the claims made in the knife world regarding convex edges are not true. Or better: they are rather meaningless. Whether a convex edge is stronger or slower dulling than a straight edges depends entirely on the angles.

However, a convex edge does have an advantage over a straight edge in that it doesn’t wedge as much. There is a smaller part of the edge in contact with the material being cut, so there is less friction.

This advantage is most prominent when cutting hard, rigid materials. That is probably the reason everyone prefers a convex edge on their axe. And cooks never complain their knife is wedging when cutting tomatoes. But they may complain when cutting potatoes.

The verdict:

A convex edge…

√  is thousands of years old

√  is marketing

×  is stronger

×  dulls slower

√  doesn’t wedge

This post is based on a discussion at the Wicked Edge forum. Many thanks to Ken, Curtis, Tom, Phil, Leo, Blunt and Clay.

2 thoughts on “What is the use of a convex edge?

  1. In fact, a convex edge that is 22 degrees at the edge of the edge dulls faster than a straight edge of 22 degrees. This is because the convex edge is thicker right behind the edge. So as the edge wears, it gets thicker faster, too. However, a convex edge that is 22 degrees at the edge of the edge dulls slower than a straight edge of 22 degrees for the same reason

    Could you elaborate? I’m thinking you made a typo in the angles.

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